Do you know when you ovulated? If not, it's very hard to say until you get your next ultrasound. Did you have another HCG test to see if the levels are increasing? That would be another thing they can do to check the viablity of your pregnancy.
As for your cervix, I believe the doctor was referring to the length of your cervix (from vagina to the uterus), and 3.5 is a good number at this stage.
I hope everything works out for you and that you get some good news at your next appointment!
6418 puts you in the 5 week range. You said your cervix was 3.5? Be more specific. Like you're dilated 3.5? I don't understand what you're even saying, I'm sorry.